There has been increasing recognition of the need to optimize sedation strategies for children with respiratory failure who are tracheally intubated and mechanically ventilated. The parent R01 Sedation Management in Pediatric Patients with Acute Respiratory Failure addresses this need with the intent to study the impact of a Nurse-Implemented Goal-Directed Comfort Algorithm on the duration of mechanical ventilation. As delineated in the grant proposal, the implementation of this algorithm will impact many factors that influence duration of mechanical ventilation, such as level of awakeness, mandated titrations and total dose of opiates and benzodiazepines. However, within each group (intervention, control) and across the entire cohort, variation in the duration of mechanical ventilation will be observed. Although this variability may correlate with the total dose of drugs administered, the ability to estimate the actual drug exposure (pharmacokinetics) may allow for a better understanding of the variability in response (pharmacodynamics). Moreover, variation in response at similar drug exposures may result from pharmacogenetically driven differences in drug action at the receptor level. Thus our hypothesis is that midazolam and morphine drug exposure and response will be affected by both non-heritable (e.g. organ dysfunction, degree of illness, age, weight) and heritable (e.g. polymorphisms in drug metabolizing systems or drug receptors) factors that can be quantitatively defined. Our long-term research goal is to improve the outcome of children with respiratory failure requiring mechanical ventilation by optimizing the sedation strategies that are used in their care. The objectives of this ancillary R01 application, which is the next step toward this long-term goal, are to 1) identify heritable and non-heritable factors that underlie the variability in the drug exposure-response to morphine and midazolam in children who are tracheally intubated and mechanically ventilated in an intensive care setting and 2) develop and validate a population pharmacokinetic(PK)-pharmacodynamic(PD) model that is predictive for the narcotic/sedative dose range requirement that provides adequate treatment yet minimizes ventilator days for children who are tracheally intubated and mechanically ventilated in an intensive care setting. This model can be ultimately be used to individualize therapy in children requiring mechanical ventilation with the goal of optimizing sedation while minimizing the duration of ventilation.